Author James Crawford had an e-book on Amazon called "Blood Soaked and Contagious", but he decided that, as a promotion, he would publish the first 3 chapters of his book on Barnes and Noble for free. So when his promotional book was published the similarity between the free B&N version and the Amazon version confused Amazon's pricing mechanism, which proceeded to list his book on Amazon as "free". By the time the problem was corrected 5,000 plus copies of his book had been given away for free and, due to Amazon's pricing policies, he is not entitled to any compensation for this error. You can read the author's account here.

Pricing a book for free on other outlets is a very common trick writers use to give their books away for free on Amazon because of Amazon's price matching policy. What writers need to remember is that the process is automated; it is a robot, a program, which makes the decisions. So if you have a book on Amazon make sure any promotional excerpts you publish elsewhere at a lower price are very obviously different from your Amazon book. Lest you confuse Amazon's pricing bot!